Concerted [2+2] cycloaddition of alkenes to a ruthenium-phosphorus double bond

Article abstract of DOI:10.1002/anie.201000356

(Figure Presented) A square deal for Ru=PR₂: The Ru=PR ₂ π bond in a terminal phosphido complex undergoes regio- and stereoselective [2+2] cycloaddition reactions of alkenes to yield metallaphosphacyclobutanes (see structure; P red, Ru yellow, C gray), which are analogous to olefin metathesis intermediates.

Full text of DOI:10.1002/anie.201000356

Angewandte
Chemie
DOI: 10.1002/anie.201000356
Metallacycles
Concerted [2+2] Cycloaddition of Alkenes to a Ruthenium–Phospho-
rus Double Bond**
Eric J. Derrah, Dimitrios A. Pantazis, Robert McDonald, and Lisa Rosenberg*
We recently described a terminal phosphido complex of
lags behind that of the corresponding amination chemistry.[3]
=
À
ruthenium containing a Ru PR2 p bond, and demonstrated
Among late-metal examples of such catalysis, the critical P C
its high activity in 1,2-addition reactions of polar and non-
polar substrates.[1] Herein we report the [2+2] cycloaddition
reactions of this planar phosphido complex with both
activated and simple alkenes to yield metallaphosphacyclo-
butanes, and we provide evidence for a concerted cyclo-
bond-forming step has been attributed either to nucleophilic
attack of free phosphine onto an unsaturated substrate that
has been activated through binding with the metal (e.g.
Scheme 1b),[4] or to attack of the strongly nucleophilic P of a
À
phosphido intermediate (M PR2) onto a substrate containing
an electrophilic carbon atom (e.g. Scheme 1c).[5] Lanthanide-
or calcium-catalyzed hydrophosphination of alkynes and
alkenes,[6–8] which involve the insertion of an unsaturated
substrate into the metal–phosphorus bond of a phosphido
intermediate (Scheme 1d), include the only examples of
metal-mediated hydrophosphination of simple alkenes.[7b,8]
Our results point to an alternative mechanism for hydro-
phosphination catalyzed by late-metals that will allow the
incorporation of a much wider range of alkene substrates
(electron rich and electron deficient) than are currently viable
in this transition metal mediated process.[9]
À
addition pathway in these P C bond-forming reactions
(Scheme 1a). Metal-mediated P C bond-forming reactions
À
The five-coordinate half-sandwich complex [Ru(h5-
indenyl)(PR2)(PPh3)] (1a,b) reacts rapidly with acrylonitrile
to give two isomers of the metallacyclic product 2a,b, in which
the nitrile group on the ruthenium-bound a-carbon atom is
oriented either syn or anti with respect to the Ru–indenyl
bond (Scheme 2). In solution, these complexes show diag-
À
Scheme 1. Types of metal-mediated P C bond formation involving
primary or secondary phosphine reagents.
represent a direct and potentially stereoselective route to
useful ligands or reagents,[2] but the development and
understanding of metal-catalyzed phosphination and
hydrophosphination by primary or secondary phosphines
Scheme 2.
nostic upfield 31P{1H} NMR signals due to the phosphorus
atom in the metallacycle (e.g., syn-2a: d = À13.3 ppm), and
upfield 13C{1H} NMR signals for the a-carbon atom attached
to Ru (e.g., syn-2a: d = À28.3 ppm). 13C{1H}DEPT-135 NMR
analysis of a mixture of syn- and anti-2a confirmed their
identical regiochemistry, showing that the a-carbon atom in
both isomers (d = À29.1 ppm in 13C NMR spectrum for anti-
2a) contains one proton and not two. Unequivocal assign-
ment of the stereochemistry of syn-2b was obtained from
solution NMR studies of a purified sample, which allowed
identification of the signal resulting from the lone proton on
the a-carbon atom, and showed its proximity to the PPh3 aryl
protons.[10] X-ray crystallography confirmed the analogous
syn structure of the major isomer of 2a in the solid state
(Figure 1).[11] We detected no trace of the other regioisomers
[*] E. J. Derrah, Prof. L. Rosenberg
Department of Chemistry, University of Victoria
P.O. Box 3065, Victoria, BC V8W3V6 (Canada)
Fax: (+1)250-721-7147
E-mail: lisarose@uvic.ca
Homepage: http://web.uvic.ca/~lisarose/
Dr. D. A. Pantazis
Institute for Physical and Theoretical Chemistry
University of Bonn, 53115 Bonn (Germany)
Dr. R. McDonald
X-ray Crystallography Laboratory, Department of Chemistry
University of Alberta, Edmonton, AB T6G 2G2 (Canada)
[**] We are grateful to the NSERC of Canada for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000356.
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Figure 1. Molecular structure of syn-2a.[11] The thermal ellipsoids are
drawn at the 20% probability level.
Figure 2. Molecular structure of exo-5a.[19] The thermal ellipsoids are
drawn at the 20% probability level
of complexes 2a,b, in which the nitrile is bound to the carbon
atom b to Ru in the metallacycle. Despite the high basicity of
P in 1a,b,[1a] we did not observe base-catalyzed acrylonitrile
polymerization in these reactions,[12] nor did we see telome-
À
rization resulting from additional insertions into the Ru C
reaction times (although still longer than that for ethylene;
Scheme 3), a qualitative indication of the rate dependence of
cycloaddition upon the concentration of alkene. The sensi-
tivity of this reaction to the steric bulk of the incoming alkene
is also highlighted by the complete absence of cycloaddition
when one equivalent of cyclooctene was added to 1a,b, even
after four weeks, by which time the samples had mostly
decomposed into 6a,b.
The participation of simple alkenes in these reactions, and
the sensitivity of rates to the size of the alkene, point to a
possible concerted [2+2] cycloaddition mechanism, as
opposed to the stepwise Michael-addition mechanism respon-
bond.[5c] Finally, we previously showed that 1a,b in solution
exists in equilibrium with a small amount of phospha-alkene/
5
=
hydride isomer (e.g., for 1b, [Ru(P{ C(CH3)2}iPr)(H)(h -
indenyl)(PPh3)]).[1] Apparently conversion of this structural
isomer into 1 occurs more rapidly than its reaction with
alkenes, since we neither see it in any of the product mixtures
described in this paper, nor do we see any products
attributable to its cycloaddition or insertion reactions.
Simple alkenes also undergo [2+2] cycloaddition reac-
À
tions at the Ru P p bond in 1a,b (Scheme 3). Solutions of 1
placed under 1 atm of ethylene rapidly and quantitatively
À
sible for other late-metal phosphido-mediated P C bond-
forming reactions.[13] Furthermore, we observe no change in
the rate of the cycloaddition of 1-hexene to 1a,b when this
reaction is carried out in a 1:1 mixture of [D8]toluene/THF,
relative to that in pure [D8]toluene.[14] If a stepwise reaction
were occurring, we would expect the rate to increase in the
more polar solvent mixture because of the stabilization of a
zwitterionic intermediate.[15] More compelling evidence for
the concerted nature of these [2+2] cycloadditions comes
from the reactions of cis- and trans-[D2]ethylene with 1a,b.
The barrier to rotation around the terminal CHDÀ group in a
putative zwitterionic intermediate would be sufficiently low
to give significant isotopic scrambling (i.e., four isomers: two
cis and two trans) if a stepwise mechanism were impor-
tant.[16,17] The reactions proceed rapidly at room temperature,
giving quantitative conversion into yellow solutions of [D2]-
3a,b that each contain just two major isotopomers in a 1:1
ratio, as determined by 31P{1H} NMR analysis (Figure 3).[18]
These results indicate that stereochemistry at the alkene is
conserved, and provide strong evidence for the importance of
Scheme 3.
transform
into
[Ru(h5-indenyl)(k2-CH2CH2PR2)(PPh3)]
(3a,b) at room temperature, as determined by 31P{1H} NMR
analysis. Similar products were obtained from the addition of
1-hexene (4a,b) and the strained internal alkene norbornene
(5a,b); the latter gave exclusive addition at the exo face (see
the Supporting Information and Figure 2). For 1:1 reactions of
these more sterically demanding substrates long reaction
times were required, which resulted in competing decom-
position to the orthometalated complexes 6a,b.[1b] The use of
excess alkene gave much less decomposition and shorter
À
a concerted cycloaddition mechanism in this P C bond-
forming reaction.
Inspection of the molecular structure of syn-2a (Figure 1)
suggests that the diastereoselectivity observed for products 2
and 4 results from crowding at Ru in these metallacycles: the
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Experimental Section
Full synthetic, spectroscopic, and computational details, as well as
X-ray crystallographic experimental details for the structures of 2a
and 5a, are available in the Supporting Information. CCDC 761575
(2a) and 761576 (5a) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.[11,19]
Received: January 20, 2010
Published online: March 31, 2010
Keywords: alkenes · cycloadditions · metallacycles · P ligands ·
.
ruthenium
Figure 3. 202.46 MHz 31P{1H} NMR spectra showing a total of four
distinct products from the addition of a) trans-[D2]ethylene and b) cis-
[D2]ethylene to 1a in [D8]toluene. [Ru]=Ru(h5-indenyl)(PPh3). Downfield
signals represent PPh3, and upfield signals represent PCy2.
[1] a) E. J. Derrah, K. E. Giesbrecht, R. McDonald, L. Rosenberg,
Organometallics 2008, 27, 5025; b) E. J. Derrah, D. A. Pantazis,
R. McDonald, L. Rosenberg, Organometallics 2007, 26, 1473.
[2] D. S. Glueck, Chem. Eur. J. 2008, 14, 7108.
proton on the metallacycle a-carbon atom points down,
toward the bulky PPh3 ligand in the syn-isomer, whereas for
anti-2 or anti-4 the more sterically demanding nitrile or butyl
group would point down. However, DFT calculations indicate
that the optimized structures of syn- and anti-2a are
isoenergetic at zero Kelvin, although thermodynamic correc-
tions lead to a DG8 value that is 5.3 kcalmolÀ1 lower for syn-
2a than that for anti-2a. Interestingly, when PPh3 is replaced
with PH3 in these calculations, which should eliminate the
anticipated steric preference for the syn isomer, the optimized
structure of the resulting syn isomer is 1.5 kcalmolÀ1 lower in
energy than the anti isomer, and thermodynamic corrections
give a DG8 value for the syn isomer that is 4.6 kcalmolÀ1
lower than that for the anti isomer. Therefore, differences in
steric crowding in the two diastereomers are small, and their
ground-state stabilities are comparable. This suggests that
transition-state effects, possibly electronic in origin,[20] dictate
the observed stereoselectivity (i.e., these are kinetic product
distributions).[21] We continue to search computationally for
the relevant transition states in these concerted cycloaddition
reactions to explain both the diastereo- and regioselectivity
observed.
[3] For examples see the following, and references therein; a) J. F.
Hartwig, Nature 2008, 455, 314; b) T. E. Muller, K. C. Hultzsch,
M. Yus, F. Foubelo, M. Tada, Chem. Rev. 2008, 108, 3795.
[4] a) P. Butti, R. Rochat, A. D. Sadow, A. Togni, Angew. Chem.
2008, 120, 4956; Angew. Chem. Int. Ed. 2008, 47, 4878; b) M. O.
Shulyupin, I. G. Trostyanskaya, V. A. Kazankova, I. P. Belet-
skaya, Russ. J. Org. Chem. 2006, 42, 17; c) A. D. Sadow, A. Togni,
J. Am. Chem. Soc. 2005, 127, 17012; d) F. Jꢀrꢁme, F. Monnier, H.
Lawicka, S. Derien, P. H. Dixneuf, Chem. Commun. 2003, 696.
[5] a) V. S. Chan, M. Chiu, R. G. Bergman, F. D. Toste, J. Am. Chem.
Soc. 2009, 131, 6021; b) D. S. Glueck, Synlett 2007, 2627; c) C.
Scriban, D. S. Glueck, L. N. Zakharov, W. S. Kassel, A. G.
DiPasquale, J. A. Golen, A. L. Rheingold, Organometallics
2006, 25, 5757; d) M. O. Shulyupin, M. A. Kazankova, I. P.
Beletskaya, Org. Lett. 2002, 4, 761; e) M. A. Kazankova, M. O.
Shulyupin, I. P. Beletskaya, Synlett 2003, 2155. In reference [5d]
the authors proposed a mechanism which includes alkene
À
insertion into a M H bond, but they later revise this interpre-
tation (Ref. [5e]) and describe the mechanism as a Michael-type
addition onto activated aryl alkenes, noting that these reactions
do not proceed for simple alkenes.
[6] K. Takaki, K. Komeyama, D. Kobayashi, T. Kawabata, K.
Takehira, J. Alloys Compd. 2006, 408, 432.
[7] a) A. M. Kawaoka, M. R. Douglass, T. J. Marks, Organometallics
2003, 22, 4630; b) A. Motta, I. L. Fragala, T. J. Marks, Organo-
metallics 2005, 24, 4995.
[8] M. R. Crimmin, A. G. M. Barrett, M. S. Hill, P. B. Hitchcock,
P. A. Procopiou, Organometallics 2007, 26, 2953.
[9] D. K. Wicht, D. S. Glueck in Catalytic Heterofunctionalization.
From Hydroamination to Hydrozirconation (Ed.: A. Togni, H.
Grutzmacher), Wiley-VCH, Weinheim, 2001, p. 143.
[10] See the Supporting Information for details. Similar data
obtained for 2a was complicated by overlap of the peaks of
interest with those from the Cy groups.
[11] Crystallographic data for 2a: C42H47NP2Ru, Mr = 728.82; crystal
dimensions (mm)0.57 ꢂ 0.28 ꢂ 0.28; monocclinic space group P21/
n (an alternate setting of P21/c [No. 14]); a = 13.1935(8), b =
There are surprisingly few examples of discrete complexes
containing metal–heteroatom bonds that undergo the inser-
tion of simple alkenes, despite the wide range of metal-
catalyzed reactions of alkenes for which the proposed
mechanisms invoke such chemistry.[22] The cycloaddition
reactions reported herein include the first examples of
intermolecular insertion of simple alkenes into a metal–
phosphorus bond, and are distinct from the lanthanide- and
calcium-catalyzed hydrophosphination described above in
[23]
À
that the additions are occurring at a M P double bond.
Currently we are investigating the conditions required to
À
16.9872(11),
c = 15.6856(10) ꢃ;
b = 91.1278(9)8;
V=
protonolyze the Ru C bond in these and related metallacyclic
ruthenium complexes to achieve a complete hydrophosphi-
nation cycle via this metathesis-like chemistry.
3514.8(4) ꢃ3; Z = 4; 1calcd = 1.377 gcmÀ3; m = 0.568 mmÀ1; l =
0.71073 ꢃ; T= À808C; 2qmax = 52.808; total data collected =
2
27646; R1 = 0.0229 (6571 observed reflections with Fo ꢀ 2s-
2
(Fo )); wR2 = 0.0632 for 415 variables and all 7203 unique
reflections; residual electron density = 0.532 and À0.247 eꢃÀ3
.
[12] F. Schaper, S. R. Foley, R. F. Jordan, J. Am. Chem. Soc. 2004, 126,
2114.
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Communications
[13] See for example Ref. [5c] and a) W. Malisch, B. Klupfel, D.
[2+2] cycloaddition within 30 min at RT (a rate intermediate
between acrylonitrile and 1-hexene), but gave the syn and
anti isomers (7a,b) in a 1:1 ratio. It is not yet clear whether this
selectivity arises from transition state effects in the original
cycloaddition (i.e. a purely kinetic phenomenon) or whether the
electron-rich nature of the resulting metallacycle hastens
equilibration of these isomers (DFT calculations again indicate
that these syn and anti isomers are isoenergetic).
Schumacher, M. Nieger, J. Organomet. Chem. 2002, 661, 95.
[14] We were limited in our choice of polar solvent because of the
reactivity of 1a,b with nitriles as well as halide-containing or
protic solvents (see Ref. [1a]).
[15] Solvent effects on rate are particularly pronounced for zwitter-
ionic intermediates in [2+2] additions, which are generally less
successful in achieving conformations that minimize the cou-
lombic separation than are those in, for example, [2+4] cyclo-
additions; a) R. Huisgen, Acc. Chem. Res. 1977, 10, 117; b) R.
Huisgen, Pure Appl. Chem. 1980, 52, 2283.
[16] Examples of the utility of [D2]ethylene in diagnosing mecha-
nisms of cycloaddition reactions include: a) P. D. Bartlett, R. A.
Minns, K. Hummel, S. P. Elliott, C. M. Sharts, G. M. Cohen, J. Y.
Fukunaga, J. Am. Chem. Soc. 1972, 94, 2898; b) K. N. Houk,
Y. T. Lin, F. K. Brown, J. Am. Chem. Soc. 1986, 108, 554; c) H. B.
Liu, R. J. Hamers, J. Am. Chem. Soc. 1997, 119, 7593; d) P. P.
Nicholas, R. T. Carroll, J. Org. Chem. 1968, 33, 2345.
[21] We have seen no evidence for the reversibility of these
[2+2] cycloaddition reactions (i.e., no free alkene appears in
solution samples of the purified metallacyclic products).
31P{1H} NMR analysis of sealed samples of 2a,b showed a
slight increase in the relative amount of the anti isomers (from
< 5% to ca. 10%) over a year at RT, consistent with extremely
slow equilibration, presumably via reversible dissociation of the
phosphine end of the metallacycle, which would allow the
requisite bond rotation(s) and inversion at Ru to give epimeri-
zation at the a-carbon atom.
[17] We also examined the addition of cis- and trans-2-butene to 1a,b,
but these internal alkenes reacted very slowly, allowing compet-
ing orthometalation of 1a,b (see Ref. [1b]). The somewhat
ambiguous results of these experiments (see the Supporting
Information) point to a stereochemical preference, in this
sterically congested system, for the formation of cisoid metal-
lacycles, which is avoided through the use of cis- and trans-
[D2]ethylene.
[22] J. W. Tye, J. F. Hartwig, J. Am. Chem. Soc. 2009, 131, 14703.
À
[23] Other relevant cycloadditions at M P double bonds include the
following. [2+1] cycloaddition of alkenes and alkynes at metal
phosphinidenes (M = PR): a) M. L. G. Borst, N. van der Riet,
R. H. Lemmens, F. J. J. de Kanter, M. Schakel, A. W. Ehlers,
A. M. Mills, M. Lutz, A. L. Spek, K. Lammertsma, Chem. Eur. J.
2005, 11, 3631; b) M. L. G. Borst, R. E. Bulo, D. J. Gibney, Y.
Alem, F. J. J. de Kanter, A. W. Ehlers, M. Schakel, M. Lutz, A. L.
Spek, K. Lammertsma, J. Am. Chem. Soc. 2005, 127, 16985; c) A.
Marinetti, F. Mathey, Organometallics 1984, 3, 456. This last
study gives evidence pointing to a concerted mechanism for this
route to metal-bound phosphiranes. [2+2] cycloaddition of
alkynes with a zirconium phosphinidene complex to give metal-
lacyclobutene complexes: d) T. L. Breen, D. W. Stephan, Orga-
nometallics 1996, 15, 5729. Stoichiometric [2+2] cycloaddition of
isocyanates and isothiocyanates with group 6 phosphenium
complexes (M = P(d+)R2): e) W. Malisch, K. Grun, O. Fey, C. A.
El Baky, J. Organomet. Chem. 2000, 595, 285; f) W. Malisch, C.
Hahner, K. Grun, J. Reising, R. Goddard, C. Kruger, Inorg.
Chim. Acta 1996, 244, 147; g) H. Pfister, W. Malisch, J. Organo-
met. Chem. 1992, 439, C11. The mechanism of these cyclo-
additions (Refs. [23e–g]) has not been studied.
[18] In the 1H NMR spectra of cis- and trans-[D2]-3a,b, peaks
resulting from the metallacycle protons show reduced multi-
plicities and intensities relative to those in the spectra of [D0]-
3a,b. 2H NMR spectra confirmed the equal distribution of
deuterium throughout all four positions on the metallacycle.
[19] Crystal data for 5a: C46H54P2Ru, Mr = 769.90; crystal dimensions
(mm)0.54 ꢂ 0.40 ꢂ 0.22; monocclinic space group P21/n (an
alternate setting of P21/c [No. 14]); a = 11.6081(7), b =
17.4813(11),
c = 19.5737(12) ꢃ;
b = 105.6999(8)8;
V=
3823.8(4) ꢃ3; Z = 4; 1calcd = 1.337 gcmÀ3; m = 0.525 mmÀ1; l =
0.71073 ꢃ; T= À808C; 2qmax = 54.988; total data collected =
2
33061; R1 = 0.0272 (7822 observed reflections with Fo ꢀ 2s-
2
(Fo )); wR2 = 0.0728 for 442 variables and all 8721 unique
reflections; residual electron density = 0.622 and À0.264 eꢃÀ3
.
[20] Addition of excess ethylvinylether, an electron-rich terminal
alkene of intermediate bulk, to 1a,b gave quantitative
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